Photolithography tools are mainly used in the manufacture of integrated circuits (ICs), flat panel displays and other micro devices. With a photolithography tool, distinct patterns formed in accurately-aligned respective reticles are successively transferred to a photoresist-coated wafer such as, for example, a semiconductor wafer or a wafer for forming a liquid crystal display (LCD). Photolithography tools are overall categorized into two types. A first type is called step-and-repeat systems (steppers), in which a reticle pattern is exposed onto an exposure area of a wafer in one shot, and the wafer is then moved relative to the reticle (or mask) to locate the next exposure area right under the reticle pattern interposed by a projection objective which helps to again expose the reticle pattern onto this another exposure area of the wafer. This process is repeated until an image of the reticle pattern is formed on all exposure areas of the wafer. The second type is called step-and-scan systems (scanners), whose exposure process is not completed by individual shots of the reticle pattern, but through a scan-and-image procedure facilitated by movement of the exposure field. During the exposure process of a scanner, the reticle and the wafer are simultaneously moved relative to the projection beam as well as the projection system, during which the reticle pattern is scanned over the entire exposure field on the wafer. All these photolithography systems need to employ suitable devices as respective carriers of the mask and wafer/substrate, which can make accurate relative movements, in order to meet the requirements of photolithography. The carriers of the mask are referred to as reticle stages and the carriers of the wafer/substrate are referred to as wafer stages.
The reticle stage of a scanner is generally consisted of a fine-motion stage and a coarse motion-stage, wherein the fine-motion stage is used for accomplishing fine and precision adjustments of the reticle and the coarse motion-stage is used for enabling large-stroke movements thereof required in the scanning exposure process, in which transferring the reticle within the photolithographic system is the most critical task, and the flatness (i.e., the deformation degree in the vertical Z direction) and positional accuracy of the reticle stage are factors that greatly determine the exposure quality.
In order to enhance the photolithography efficiency, some scanners, for example, those equipped with split lenses and used in the production of flat panel displays, expand the exposure field by the use of large reticles. For instances, the reticle size for Generation 4.5 (G4.5) to Generation 6 (G6) production lines is generally 520×610 mm or 520×800 mm, and that for production lines of the later generations is generally 850×1200 mm, 850×1400 mm, or larger. In addition, in line with their expanded areas, these reticles are generally made with a thickness of 8 mm. However, such large reticles tend to experience deformations when they are lifted by suction, causing horizontal irregularities and deviations in the Z direction, some of which are up to 50 μm in some cases. Serious deterioration in imaging performance will occur if such adverse impact is not properly controlled.
To address this problem, those skilled in this art have employed photoelectric detection systems to real-time measure the Z direction deviations of horizontal positions of reticles, thus can achieve real-time control of objective imaging plane using the measurements as inputs. However, this solution is complex and costly and imposes high requirements on the projection objective. An exposure apparatus has also been proposed which employs multiple sites for vacuum attraction of a reticle and for correcting a deformation of the reticle by adjusting the vacuum pressures. However, this apparatus is associated with the problem that, as the sites are in fixed positional relationship with respect to the reticle, the negative pressures cannot be attained if the deformation is too large and forms a great gap between the reticle and attraction sites.